Identify the mechanism by which each of the reactions above proceeds from among
ID: 971447 • Letter: I
Question
Identify the mechanism by which each of the reactions above proceeds from among the mechanisms listed. Use the letters a- i for your answers.
Identify the mechanism by which each of the reactions above proceeds from among the mechanisms listed. Use the letters a- i for your answers.
Identify the mechanism by which each of the reactions above proceeds from among the mechanisms listed. Use the letters a- i for your answers.
a = Proton transfer d = Electrophilic addition g = SN1 Nucleophilic substitution b = Lewis acid/base e = E1 Elimination h = SN2 Nucleophilic substitution c = Radical chain substitution f = E2 EliminationExplanation / Answer
I.
1. Reaction of 1,2-dibromocyclohexane with potassium tertiary butoxide to form cyclohexadiene
The product (cyclohexadiene) shows absence of two hydrogen atoms and two bromine atoms that were present in reactant (1,2-dibromocyclohexane), and the product contains two double bonds. This indicates that the reaction is elimination reaction.
E1 elimination is favoured when,
a)it is possible to form a more stable carbocation such as tertiary carbocation/ benzyl cation on loss of leaving group
b) base is a weak base and c) a highly polar solvent is used
The above reaction takes place largely by E2 elimination mechanism as it involves a strong bulky base (potassium tertiary butoxide).
So the answer is f = E2 elimination
2. Reaction of tertiary pentyl chloride with methanol to form tertiary pentyl methyl ether
The product (tertiary pentyl methyl ether) contains methoxy (-OCH3) group in place of the Cl group that was present in reactant. Hence, it is a substitution reaction. Since no radical initiator or any radical conditions(suitable light and reagents like peroxides) is used, the reaction is NOT a radical substitution reaction. It is a nucleophilic substitution reaction as Cl- is substituted with CH3O- .
Since the reactant is a tertiary chloride the reaction takes place by SN1 mechanism.
The answer is g= SN1 Nucleophilic substitution.
II.
1. Reaction of methyl cyclopentyl chloride to methylcyclopentyl ammine in presence of excess of ammonia
The product (methylcyclopentyl ammine) contains amino (-NH2 ) group and lacks Cl group that was present in reactant. Hence it is a substitution reaction. No radical conditions are used and so its not a radical reaction.
The methyl and amino groups are not on same side of the ring in product while methyl and chloro groups are on same side of ring in reactant. Hence, nucleophilic substitution at the chloro containing carbon has taken place from below the ring and not from same side as that of leaving group. There is inversion of configuration. Hence, the reaction has taken place by SN2 mechanism.
Answer is h = SN2 Nucleophilic substitution
2. Reaction of propylene oxide with n-propyl amine
The reaction is a nucleophilic substitution reaction as one of the C-O bonds in reactant is replaced by C-N bond in product. The reaction has taken place by SN2 mechanism. If the reaction would have taken place by SN1 mechanism then the epoxide would have first opened such that carbon containing methyl group forms a more stable secondary carbocation which woud then be attacked by the amine but this is contrary to the product actually formed in which nitrogen is not bonded to methyl group containing carbon butbis bonded to the other carbon.
Hence, answer is h = SN2 Nucleophilic substitution
III.
1. Reaction of secondary pentyl chloride with potassium tertiary butoxide to form pentene
The products (pentenes) have double bond and lack H, Cl atoms that were present in the reactant (secondary pentyl chloride). Hence, its elimination reaction. Since the reaction takes place in presence of strong bulky base namely potassium tertiary butoxide the mechanism is E2 elimination.
Hence, the answer is f = E2 Elimination
2. Reaction of di-isopropylamine with n-butyl lithium to form Lithium di-isopropylamide and butane
The reaction involves proton transfer. It is an acid-base reaction. Di-isopropylamine which acts as proton donor is Bronsted acid and n-butyl lithium which accepts proton is the Bronsted base.
So, the answer is a = Proton transfer